PGP?
I don't know anything about it. I am not sure if PHP can implement it.
http://www.pgp.com/ that is a company that works with this stuff. I do not know of any freely available encryption packages, if they were freely available people would be freely available to decrypt any of the incripted messages.
They did not hire you to be a computer mathematician, so expecting you to come up with a custom encryption package would be silly.
You could do it via a text replace function for all the ascii characters.
if this character replace with this character. Then when they recieve it they do the reverse, if this character replace with this character, you would only need to do this with the subject as well as the message body.
That is a very simple encryption scheme.
set a = z, b = y, c = x etc, for example. Then on the recipients they would set z = a, y = b, x = c and wallah decrypted. Not too hard a task to implement. But it is still not very secure, if you want it to be secure you will have to hire a computer mathematician that could work out a powerful encryption scheme for you, or go out and buy one from a professional service.
I have heard that PGP keys work well, for email security, I saw something about PGP on woodmann.com/fravia in his mail me section http://www.woodmann.com/fravia/info.htm at the bottom you will see a PGP encrytion scheme, I don't know how it works, but it is for secure transactions. I wouldn't mail him about it, because he may hack you, or worse (he doesn't like to be asked questions with answers that can be found else where on the web).
Good luck with this, I think you will need it.
-mage-